As an integral part of our structural studies of proteins we routinely need to oxidatively fold polypeptides. Both synthesized peptides and proteins cytoplasmically expressed in vivo do not form disulfide bonds. In a number of cases our lab is intensively working out the in vitro conditions necessary for formation of the native disulfide bonding pattern. Analysis of these efforts would be greatly accelerated by the ability to quickly and accurately determine the weight of the products of such reactions. This information will allow us to watch the removal of blocking groups (on synthesized polypeptides), check for unwanted labeling of labile residues withoxidative agents (such as iodine), and, if possible, determine the relative amounts of SH and S-S groups.